M43photo made a very interesting Tele Converter test on the GH2 and 14-42mm lens. The ETC mode gives a tele conversion of 2.6x when recording 1080 lines in and 3.9x in 720 lines (Video mode).
As you can see from the image samples on the website: “It appears that the video stream recorded using ETC has worse image quality than the normal stream, at all ISO values. There is some lack of sharpness and contrast, and more noise in the ETC video streams.”
M43photo explains the (little) quality loss with the fact that ETC recording doesn’t have the option of scaling down from a larger number of pixels (in normal recording mode the whole sensor area is used and scaled down).
I suggest you to make your own tests. Let us know if you can confirm the conclusions from m43photo! Thanks!
UPDATE: A good friend of us (and very good tester) said:
The extender 2,6 function depends on the optic that is used. The Leica 45mm Macro[/shoplink] gives the same frame quality in extender mode as a HI Q optics. The same for 7- 14 mm. Both tested result! The 14- 42 is the weakest lenses in the Panasonic lens set. So the result can be right but the result is not easy to apply on other optics“